Chemical, petrochemical, and pharmaceutical processes usually require bringing reactants (the components to be mixed) into close contact by imposing a mixing flow. Mixing is a complicated phenomenon, which even in the simplest case of mixing miscible fluids, involves a combination of three non-linearly coupled, spatially-distributed processes: convection, stretching and diffusion. Convection moves portions of material from one location to another, promoting global uniformity by redistributing initially segregated components. Stretching transforms portions of material into elongated striations, increasing the amount of contact area. Diffusion induces uniformity at small scales. These processes typically generate partially mixed structures that exhibit strong variability in local composition. Chemical reactions taking place in this inhomogeneous environment often exhibit spatially dependent rates.
The stirred tank reactor is the most common type of process equipment used to conduct mixing and chemical reactions in a wide variety of industries. Stirred tanks are versatile, and commercially available in a wide variety of sizes, impeller designs and baffle configurations. They can be used for both continuous and batch processes, they can handle reactant volumes from a few gallons to many thousands of gallons per hour, and they have been successfully used to process single liquid phases as well as liquid-liquid, gas-liquid, and solid-liquid dispersions.
However, stirred tanks have some limitations related to flow patterns within them see, e.g., F. O'Connell et al, Chem. Eng. Prog., 46:358-362 (1950); H. Kramers et al, Chem. Eng. Sci., 2:35-42 (1953); R. Biggs, AIChE J., 9:636-640 (1963); K. Norwood et al, AIChE J., 6:432-436 (1960); S. Aiba, AIChE J., 4:485-489 (1958); A. Metzner et al, AIChE J., 6:109-114 (1960); L. Dong et al, Chem. Eng. Sci., 49:549-560 (1994); A. Desouza et al, Can. J. Chem. Eng., 50:15-23 (1972); C. Kuncewicz, Chem. Eng. Sci., 47:3959-3967 (1992); and C. Perng et al, "A Moving-Deforming-Mesh Technique for Simulation of Flow in Mixing Tanks", in Process Mixing: Chemical and Biochemical Applications --Part II, G. B. Tatterson et al (eds), AIChE Symp. Series (1993)!. Segregated regions which form in mixtures act as barriers to mixing, substantially increasing both the mixing time and the amount of by-products generated in industrial operations.
For components having Reynolds numbers (Re) less than 500, mixing in stirred tanks is often inefficient and characterized by persistent, well defined ring vortices above and below the impeller. See, e.g., J. Sachs et al, Chem. Eng. Prog., 50:597-603 (1954); Metzner, cited above; M. Yianneskis et al, J. Fluid Mech., 175:537-555 (1987); J. Costes et al, Chem. Eng. Sci., 43:2751-2764 (1988) and Dong, cited above!. Contrary to the common assumption that segregation regions are readily destroyed by baffles, many of these studies have reported the persistence of well defined toroidal vortices, both in baffled and unbaffled vessels.
Although such segregated regions strongly affect the performance of reactive processes, little information about these regions has been reported in the literature, nor solutions focusing on the problems caused by the location and size of the segregated regions, nor methods for preventing their formation.
Such problems in mixing of industrial, chemical, or pharmaceutical components or reactants have both economic and technical consequences. Numerous processes of interest to industry involve mixing high viscosity materials. Examples include blending of molten polymers, reactive polymerization processes that use heterogeneous catalysts, mixing of fiber suspensions, etc. In these, as in many other processes, good mixing is often a necessary condition for process success. Mixing is sometimes inefficient, i.e., for fast reactions or viscous fluids, mixing is often slow compared to the rate of reaction. Such inefficiency in mixing may result in the slowing or even halting of desired reactions before reaching completion, the unintended enhancement of undesired reactions, and a decreased product selectivity.
The most common method for achieving homogeneity for low viscosity fluids is to use a high stirring rate, ideally inducing a turbulent flow. Unfortunately, such an approach is impractical in processes involving high viscosity fluids typical in polymerization reactions, because increases in stirring rates lead to huge increases in energy demands and the stresses required to achieve turbulent flow often exceed equipment capabilities. Moreover, such stresses can damage the materials being mixed.
In addition, for shear-thinning materials (e.g., many polymers), stronger stirring can increase segregation. Fast stirring is also impractical in many biotechnological applications where materials are shear sensitive (proteins and other macromolecule, fibers, cellular materials) and high-speed spinning at high shear rates often lead to widespread damage to proteins and other macromolecules.
There is a need in the art, therefore, for efficient methods and compositions for achieving fast and efficient mixing of high viscosity fluids in agitated tanks of fixed design under gentle or slow stirring conditions.